In this paper we focus on how South African students' ideas about the human body are constituted in their descriptions of three different scenarios involving the pathway of a sandwich, a painkiller and a glass of water through the body. In particular, we have studied the way in which the students transferred ideas between the sandwich and the painkiller compared with the students' ability to explain the water pathway. The study surveyed 161 ninth-grade students in five different schools in South Africa. Data collection methods used were: drawings, written questions (open-ended items) and interviews with selected students. The questions emerged from the three scenarios--what happens in the body when you eat a sandwich, swallow a painkiller and drink a glass of water. We report that it is difficult for the students to transfer knowledge of the digestive system horizontally from the sandwich scenario to descriptions of the painkiller and water scenarios. The integration of three organ systems (digestive, circulatory and excretory) to describe the water scenario was even more difficult for the students than the horizontal transfer from the sandwich scenario. The students also showed a diversity of non-scientific descriptions, especially concerning the water scenario. The paper discusses why a large percentage of the students (~50%) included non-scientific ideas in their descriptions of the water scenario.